Audio systems have become increasingly popular in recent years, and much development effort has been directed toward improving the quality and integrity of their audio imaging. One important aspect of audio imaging is the creation of a sound field in which a listener perceives depth and directional qualities in the sound field created by a plurality of sources.
One example of a multi-channel audio system which provides an enhanced sound field is described in U.S. Pat. Nos. 3,632,886, 3,746,792, and 3,959,590, all invented by Scheiber. In Scheiber's system, as many as four audio channels may be encoded to two channels for recording or transmission and decoded at playback to produce multiple channels (typically four) of audio information. In this type of system, speakers or transducers are placed peripherally around a listener to produce a sound field in which sound may be perceived as originating from substantially any direction.
Another example of a multi-channel audio system which provides an enhanced sound field is the well known "surround sound" audio system designed by Dolby Laboratories. In this system, multiple channels of audio information are also encoded to two channels for recording and decoded at playback to produce a multi-dimensional sound field. In this system, the primary sound sources are located in front of a listener and secondary sound sources are disposed peripherally around a listener to create the desired directional effects. This system is particularly popular for use with the audio portions of motion pictures.
Still another multi-channel sound system is described in U.S. Pat. No. 4,478,167, invented by Borkin. Borkin teaches a three channel sound system in which speakers are located in a triangular pattern around a listener. In Borkin's system, a center channel signal is derived by summing a portion of left side channel signal and a portion of the right side channel signal. In addition, a portion of the right side signal is cancelled from the left side channel and a portion of the left side signal is cancelled from the right side channel. According to Borkin, the proportions of the amount of side channel cancellation range from approximately 2/3-3/4 to achieve the desired results. In Borkin's system, the gain of each of the audio channels is identical and fixed, thus requiring transducers of equal size wherein the placement of the transducers relative to the listener is critical.
While each of the above systems provides an enhanced sound field in a spacious environment such as a home living room or movie theater, they are not particularly useful for use in an automotive environment. As exemplified by the systems noted above, much development effort has been directed toward bolstering the directional information present in a sound field and the above systems function quite well in installations where sound sources and listeners can be positioned in optimal locations. However, in an automotive environment, the location of listeners relative to sound sources cannot be readily adjusted. For example, sound sources in automobiles are typically placed in doors, side panels or rear decks, and once installed, cannot be moved. The position of the listeners, in this case a driver and one or more passengers, is necessarily fixed by the location of seats within the automobile. If the sound system is adjusted to produce a balanced sound field proximate either the driver or passengers, the sound field will be unbalanced near the other occupants of the automobile. No system is known which allows a sound field to be optimized for one occupant of an automobile while also providing an optimally balanced sound field for the other occupants of an automobile. Furthermore, no system is known which generates a balanced sound field in a center channel sound system wherein the components used in the center channel may be smaller than the side channel components and further wherein the placement of the center channel transducer is not critical.